Incremental taking almost as long as full

Hoping someone can shed some light on an issue we are having. We have a large amount of data sat on a file server that is being DFS replicated around the world. Our goal is to actually get it backed up to a tape.

Problem number 1: running a backup of a DFS share (via Shadow Copy Components) takes around 10 times as long as a backup of the same data that has been copied (so just a folder backup)

Problem number 2: we are planning on running full and incremental backups, but the incremental backup is taking almost as long as the full!

Test data: 3GB, DFS "full" backup take 50 mins, "incremental" takes 43 mins backing up 1.2MB. The same data in a folder takes 3 mins to backup full and 2 for incremental.